1. Field of the Invention
The present invention relates to a heatsink arrangement for a semiconductor device used in electric/electronic circuits.
2. Description of the Related Art
A semiconductor device used in an electric/electronic circuit generates heat due to power loss in the semiconductor device. Such a semiconductor device is typically provided with a heatsink for preventing the semiconductor device from breaking down due to heat generated therein. In a linear power amplifier for amplifying a small-power signal, e.g., an audio signal, into a large-power signal, a large amount of heat is generated due to power loss in the semiconductor device, and such a semiconductor device requires a heatsink with a large surface area and a large volume. A heatsink of a type that is placed in close contact with a semiconductor device is typically made of a metal such as aluminum, and radiates heat that has been generated in the semiconductor device.
An amplifier for amplifying a pulse-width-modulated small-power signal with a semiconductor switching device (typically a transistor or a MOSFET) generates less heat than a power amplifier as described above. However, with a high-output power amplifier, a large current flows through a semiconductor device performing the switching operation, thereby increasing the amount of heat generated due to power loss in the semiconductor device. Therefore, such an amplifier requires a heatsink.
Capacitive coupling occurs between a semiconductor device and a metal heatsink, which are placed in close contact with each other. As the semiconductor device performs the switching operation at a high frequency, high-frequency noise occurring due to a large current flows as a noise current into the heatsink via the capacitive coupling, and the high-frequency noise is radiated from the heatsink with a large surface area and a large volume functioning as an antenna. The radiated high-frequency noise should be reduced as it may adversely affect other electronic devices.
In the conventional art, a thermally-conductive spacer is provided between a semiconductor device and a heatsink to increase the distance therebetween and thus to reduce the capacitive coupling therebetween, in order to reduce high-frequency noise radiated from the heatsink. This however lowers the heat-radiating efficiency, and there is a certain limit to how much the distance between a semiconductor device and a heatsink can be increased by providing a spacer therebetween.
Another approach in the conventional art is to electrically connect a heatsink with a chassis of an electronic device so that a noise current flowing into the heatsink is passed to the grounded chassis, thereby reducing the radiation of high-frequency noise. According to still another approach in the conventional art, a dielectric material is provided between a semiconductor device (CPU) and a heatsink, while connecting the heatsink and the chassis of the electronic device with a conductive connection line. With the provision of the dielectric material, the semiconductor device and the heatsink are actively coupled together in capacitive coupling so as to flow the high-frequency noise current from the heatsink to the chassis (see pp. 1-3 and FIG. 1 of JP2853618B).
With such a heatsink arrangement, however, the high-frequency noise current flowing through the heatsink and the chassis increases as the current flowing through the semiconductor device increases. As a result, the flow of the noise current from the heatsink to the chassis via the connection line forms a mechanically large loop passing through the connection line and the chassis, thereby radiating substantial high-frequency noise. Thus, with the conventional heatsinks and heatsink arrangements, it is not possible to sufficiently reduce the high-frequency noise radiated by the heatsink.